This invention is generally in the field of electrical measurements and relates to a method and system for measuring a thickness of a conductive film. The present invention is particularly useful for measuring conductive layer bearing structures for contactless electrical testing. The technique of the present invention can be used for controlling a certain process of the sample manufacture, for example manufacture of semiconductor wafers.
The manufacture of semiconductor devices typically includes a process of depositing a metal layer onto a semiconductor wafer in order to define interconnects. The quality of this process, as well as that of a process of removing metal from selected regions (e.g., polishing) that follows the deposition process, should be controlled.
FIG. 1 illustrates a cross section of a copper-based wafer structure 10 (utilizing copper interconnects patterned with a known dual Damascene process) prior to the application of a CMP process to the structure. The structure 10 includes a substrate 11 with an inter layer dielectric (ILD) 12 thereon, optional xe2x80x9cetch stopxe2x80x9d layer 14 (e.g., SiN), ILD layer portions 16 and 18, and a copper layer 20. These stack layers define a dense structure 22, which is composed of the ILD layer portions 18 and copper layer 20, and is surrounded by the ILD field layer portions 16.
Copper is deposited by one of the known techniques, such as CVD, PVD electroplating or electroless plating. Depending on the deposition process, the uppermost copper layer 20 has certain topology, namely, has the topology within the dense structure 22 repeating that of the underlying pattern.
It should be noted that, if electroplating is used, a thin copper seed layer 24 (with the thickness of about 1000-5000 A) should be deposited onto the structure prior to the deposition of the layer 20 as a prerequisite for electroplating. The thickness of this layer could be measured by optical- or electrical-based techniques. Additionally, although not specifically shown, a barrier layer (TaN or Ta) is typically provided above the ILD layer portions 16 and 18 to prevent copper migration therein.
After deposition, a chemical-mechanical polishing (CMP) is applied in order to remove copper from the top ILD surface and thus to leave copper only within the ILD trenches.
Copper CMP is a complex process because of the need to completely remove the barrier layers and copper, without the overpolishing of any feature. This is difficult because current copper deposition processes are not as uniform as the oxide deposition process. To this end, the quality of the copper deposition process should be controlled.
Contact electrical-based measurement techniques have been developed and are disclosed, for example, in the scientific article xe2x80x9cAn Overview of Thickness Measurement Techniques for Metallic Thin Filmsxe2x80x9d, S. C. P. Lim and D. Ridley, Solid State Technology, February 1983, pp. 99-103.
It is also known to use an eddy current passage through a conductive film for contactless measurement of the film properties, such as conductance/resistance and thickness. Such a technique is disclosed, for example, in. U.S. Pat. No. 4,849,694. This technique utilizes an eddy current apparatus including an alternating frequency driving coil, a detector coil mounted in a housing adjacent one surface of the thin film, and circuitry for measuring the signal across the detector coil which senses the field after it is subjected to the eddy currents generated within the conductive film. Precise adjustment of a fixed distance between coils and film surface is important and achievable by positioning the film surface at the focal point of an optical microscope objective lens to which the eddy current apparatus is coupled.
Another techniques of the kind specified are disclosed in the following patent publications:
WO 01/46684 discloses in-situ metallization monitoring using eddy current measurements and optical measurements that can be integrated in a CMP machine. According to this technique, an eddy-current sensor (ESC) and an optical reflectivity sensor are embedded in the polishing table. The eddy-current sensor (ECS) operates with relativity low frequency (up to 100 MHz). A measuring scheme utilizes two coils in balanced bridge, a differential amplifier, a synchronous detection (in-phase and in-quadrature). The optical sensor simply measures the reflection from the wafer under measurements: polishing end point corresponds with a dip in reflectivity graph (vs. time).
U.S. Pat. No. 6,433,541 discloses a method and apparatus of obtaining information in-situ regarding a film of a sample using an eddy probe during a process for removing the film. The eddy probe has at least one sensing coil. An AC voltage is applied to the sensing coil(s), and one or more first signals are measured when the sensing coil(s) are positioned proximate the film of the sample, and one or more second signals are measured when the sensing coil(s) are positioned proximate to a reference material having a fixed composition and/or distance from the sensing coil. The first signals are calibrated based on the second signals so that undesired gain and/or phase changes within the first signals are corrected. A property value of the film is determined based on the calibrated first signals.
U.S. Pat. No. 6,407,546 discloses a non-contact technique for using an eddy current probe for measuring the thickness of metal layers disposed on semi-conductor wafer products. This technique utilizes a probe housing, comprising an eddy current sense coil and a linear motion controller, and a computer that controls the linear motion controller and the eddy current sense coil. The computer identifies a thickness of the inspection sample by a method comprising the generation of a natural intercepting curve based on resistance and reactance measurements of at least two data points. Then, a plurality of corresponding resistance and reactance measurements of a location on the inspection sample is obtained with the eddy current sensor, where the eddy current sensor makes a first measurement at a first distance from the inspection sample, and makes each of the remaining plurality of measurements at a distance that is incrementally further away from the inspection surface. Next an inspection sample curve is generated based on the plurality of corresponding resistance and reactance measurements obtained from the inspection sample. An intersection point between the natural intercepting curve and the inspection sample curve is also generated. A vector impedance for each of the at least two data points, and the intersection point, is calculated to identify a closest two data points that the intersection point is positioned. Then, the thickness of the identified location of the inspection sample is calculated by performing an interpolation between the closest two data points.
U.S. Pat. No. 5,552,704 discloses an eddy current test method and apparatus for measuring conductance. According to this method, an eddy current probe is used, without the need for measurement or knowledge of the separation between probe and sample. The probe comprises sense and drive coils mounted in close proximity to each other (or a single coil which functions as both a sense and drive coil), circuitry for producing AC voltage in the drive coil, and a meter for measuring in-phase and quadrature components of induced voltage in the sense coil. Look-up table data can be generated for use in subsequent measurements on samples of unknown conductance by performing eddy current measurements on samples having different known conductances to generate reference lift-off curves, processing the reference lift-off curves to determine a conductance function relating each known conductance to a location along a selected curve, and storing conductance values determined by the conductance function for different points on the selected curve as the look-up table data. An unknown sample conductance can then be determined by generating a lift-off curve from voltage measurements at different probe separations from the sample, determining a new intersection voltage pair representing the intersection of the lift-off curve with the selected curve, and determining the unknown conductance as a look-up table value indexed by the new intersection voltage pair.
U.S. Patent Publication No. 2002/0053904 discloses an apparatus for measuring the thickness of a thin conductive film formed on a substrate. The apparatus of the invention includes an eddy current coil sensor, disposable at a predetermined position near a conductive film, for generating a predetermined eddy current in the conductive film and for detecting a magnetic field caused by the eddy current. The apparatus also includes a displacement sensor for measuring a displacement between the eddy current coil sensor and the conductive film. The thickness of the conductive film is measured in accordance with a variation in inductance of the eddy current coil sensor and the amount of displacement measured by the displacement sensor.
U.S. patent publication No. 2001/0008827 discloses a polishing apparatus, wherein a conductive layer is polished while the surface of the substrate is brought in sliding contact with the polishing surface. A sensor, which typically comprises an eddy-current sensor, passes directly below the surface, being polished, of the substrate each time the polishing table makes one revolution. Since the eddy-current sensor is positioned on an arcuate path passing through the center of the substrate, the eddy-current sensor is capable of continuously detecting the thickness of the conductive layer as the eddy-current sensor moves along the arcuate path beneath the substrate.
U.S. patent publication No. 2002/0047705 discloses an eddy current sensor comprising a sensor coil for generating an eddy current in the conductive film, and a detector for detecting a change in the thickness of the conductive film from a change in a resistance component in impedance formed by the sensor coil and conductive film.
U.S. patent publication No. 2002/0077031 discloses a combination of eddy current sensor and optical sensors. The eddy currents cause a metal layer to act as an impedance source in parallel with a sense coil and capacitor. As the thickness of the metal layer changes, the impedance changes, resulting in a change in the Q-factor of sensing mechanism. By detecting the change in tie Q-factor of the sensing mechanism, the eddy current sensor can sense the change in the strength of the eddy currents, and thus the change in thickness of the metal layer.
U.S. patent publication No. 20010054896 discloses an eddy current based measuring technique, according to which the thickness or strength of a flat test object, in particular a web, a tape, or a layer of an electrically conducting material, is determined with the aid of at least one measuring coil through which an alternating current passes, wherein the measuring coil is arranged at a basic distance from the test object. The change of inductance and damping are evaluated via the impedance.
The prior art teach that film thicknesses of more than 0.1 xcexcm are measurable with less than 100 MHz frequencies of AC voltage applied to a sensing coil (e.g., is the above-indicated U.S. patent publications Nos. 2001/0008827 and 2002/0047705. Most of the prior art techniques utilize means for determining a distance between the sensor and the samplexe2x80x94an important parameter that affects the measurement results.
There is accordingly a need in the art to facilitate contactless electrical measurements of conductive film thicknesses by providing a novel electrical measurement method and system.
The technique of the present invention provides for thickness measurements utilizing high frequencies (from 100 MHz to a few GHz) AC voltage applicable to a small sensing coil (e.g., RF coil fabricated by integrated technology). Such eddy-current measurements at high frequencies provide for decreasing the penetration of an electromagnetic wave into the film material and thus decreasing the influence of a sample holder (chuck), substrate (wafer or another semiconductor or dielectric material), and/or under-layers and structures formed from the conducting media (metals or semiconductors). Appropriate frequency or frequency range can be chosen by calculation of appropriate skin-depth taking into account interaction and attenuation of high-frequency electromagnetic wave passing through the structure (stack).
Utilization of high frequencies causes in turn the decrease in the signal changes related to the eddy-current effect. The conventional eddy-current techniques for measuring of thin films in the thickness range required for semiconductor industry (0.1-2 xcexcm) utilizes relatively low frequenciesxe2x80x94from several MHz to tens of MHz (US2002/0047705).
The present invention provides an alternative method for measuring required metal films (0.1-3 xcexcm in thickness) by locating a high-frequency eddy-current probe (from 100 MHz to a few GHz) in close proximity from the sample, i.e., from several tens to several hundreds of micrometer. Due to a significantly decreased sample-to-probe distance, it is possible to measure eddy-current signals in the high-frequency range with required resolution and accuracy. Due to significantly increased frequency, the penetration of electromagnetic waves to the under-laying structures, substrate and wafer holder decreases, thus decreasing the sensitivity to undesirable layers and materials below the film and increasing xe2x80x9cselectivexe2x80x9d sensitivity to the measured film only. Operation with increased frequencies allows for using smaller inductances for eddy-current coil and thus smaller coils improving lateral resolution of the probe. Commercial high-frequency (UHF and microwave range) coils with reduced dimensions can be used. This resolution in turn can be improved by designing high-frequency coils with magnetic-field concentrator from high-frequency ferrites.
The technique of the present invention provides for eliminating the need for controlling a distance between a sample under measurements and a measuring coil, but rather allows for measurements independent on the value of distance. The present invention also provides for correcting the measurement results for the environment effects (e.g., temperature effects). The technique of the present invention utilizes for selecting an appropriate model for analyzing measured data as well as for optimizing this model while at a calibration step (off-line).
There is thus provided according to one broad aspect of the present invention, a method for measuring in an electrically conductive film of a sample, the method comprising:
providing data indicative of a free space response of an RF sensing coil unit to AC voltage applied to the RF sensing coil;
locating said sensing coil proximate to the sample at a distance h substantially not exceeding 0.2 r wherein r is the coil radius; supplying an AC voltage in a range from 100 MHz to a few GHz to the sensing coil thereby causing generation of an eddy current passage through the conductive film; detecting a response of said sensing coil to an effect of the electric current through the conductive field onto a magnetic field of the coil and generating measured data indicative of said response; and analyzing the measured data and said data indicative of the free space measurements to determine at least one of thickness and resistance parameters of the conductive film,
the method providing for measuring in conductive films with a sheet resistance R5 in a range from about 0.009 to about 2 Ohm/m2.
Preferably, the AC voltage frequency ranges between 200 MHz to 500 MHz. For copper films, measurable thicknesses are about 0.1-3 xcexcm (of the skin depth order).
The utilization of high-frequency eddy-current technique requires solving such important problems, as providing an appropriate high-frequency measurement technology. (Frequently used very sensitive balanced bridge technique is difficult to realize at high frequencies even in the single-frequency mode, in frequency sweep mode, an appropriate microwave measurement technology is required); appropriate physical model of interaction between the sample (conductive film) and an eddy-current probe; appropriate algorithms for data acquisition and processing, calculation and interpretation; appropriate calibration and measured procedures; appropriate distance control technique to ensure well-controlled and reliable work of the system without any potential danger to very expensive semiconductor wafers.
As indicated above, the technique of the present invention provides for the film thickness measurements independent on a distance between the sensing coil and the film, thereby eliminating the need for measuring this distance.
One way to achieve this is to select for measurements in a given sample, by frequency sweeping an optimal value of the AC voltage frequency for said, wherein this optimal value is that at which a characteristic of the response signal is substantially independent from a value of the distance between the coil and the sample. The optimal value can be determined by carrying out a calibration stage. The calibration stage may comprise the following: determining a frequency characteristic of the coil with no sample in the vicinity of the coil (free space response of the coil); varying the frequency value and determining a value of the coil response as a function of the frequency; and analyzing the frequency characteristic and the coil response function to determine the optimal frequency value for the given sample. Preferably, the coil response value is the real part of the coil circuit impedance obtained by transformation from the reflection response (S11).
Another way to achieve the distance independence of measurements consists of the appropriate model selection and optimization of the selected model during a calibration stage (off-line). Here, either a fixed frequency or multi-frequency approach for the model optimization can be used.
According to yet another embodiment, a ratio between inductance L and resistance R of an equivalent RLC circuit (defined by the coil and sample arrangement) is considered, which is proportional to the thickness of the conductive film.
According to another broad aspect of the present invention, there is provided a method for measuring in a copper film of a sample, the method comprising:
providing data indicative of a free space response of an RF sensing coil unit to AC voltage applied to the RF sensing coil;
locating said sensing coil proximate to the sample at a distance h substantially not exceeding 0.2 r wherein r is the coil radius; supplying an AC voltage in a range from 200 MHz to 500 MHz to the sensing coil thereby causing generation of an eddy current passage through the conductive film; detecting a response of said sensing coil to an effect of the electric current through the conductive field onto a magnetic field of the coil and generating measured data indicative of said response; and analyzing the measured data and said data indicative of the free space measurements to determine at least one of thickness and resistance parameters of the conductive film,
the method providing for measuring in copper films with a thickness d in a range from about 0.01 xcexcm to about 3 xcexcm.
According to another broad aspect of the present invention, there is provided a system for measuring in an electrically conductive film of a sample, the system comprising:
an RF sensing coil operable to generate a magnetic field thereby causing an eddy current passage through the conductive film located proximate to the coil;
an AC voltage generator connected to said sensing coil, said AC voltage generator being operable to generate AC voltage of a frequency ranging between 100 MHz and a few GHz;
a detector, which is connected to the sensing coil and operable to detect an effect of the electric current passage through the conductive field onto a magnetic field of the coil and generate a corresponding response signal;
a control unit connectable to the AC voltage generator to operate it and connectable to the detector to receive and analyze the response signal, the control unit operating said AC voltage generator to provide at least two operating frequencies of the AC voltage within said range and thereby determining a value of said response as a function of the frequency, and utilizing calibration data to analyze said function and determine at least one of thickness and conductivity of the conductive film.
The technique of the present invention can advantageously be used in the manufacture of semiconductor devices for feed-back and feed-forward control of such processes as electroplating, CMP, and PVD, as well as general mapping (one dimensional or two-dimensional) of the topography of a patterned layer. This may for example be used for die mapping in a semiconductor wafer. The present invention also provides for automatic recipe design to be further used in electrical measurements of the layer thickness.